7.20.3 SSAX: A Functional XML Parsing Toolkit

7.20.3.1 History

Back in the 1990s, when the world was young again and XML was the
solution to all of its problems, there were basically two kinds of XML
parsers out there: DOM parsers and SAX parsers.

A DOM parser reads through an entire XML document, building up a tree of
“DOM objects” representing the document structure. They are very easy
to use, but sometimes you don’t actually want all of the information in
a document; building an object tree is not necessary if all you want to
do is to count word frequencies in a document, for example.

SAX parsers were created to give the programmer more control on the
parsing process. A programmer gives the SAX parser a number of
“callbacks”: functions that will be called on various features of the
XML stream as they are encountered. SAX parsers are more efficient, but
much harder to user, as users typically have to manually maintain a
stack of open elements.

Kiselyov realized that the SAX programming model could be made much
simpler if the callbacks were formulated not as a linear fold across the
features of the XML stream, but as a tree fold over the structure
implicit in the XML. In this way, the user has a very convenient,
functional-style interface that can still generate optimal parsers.

The xml->sxml interface from the (sxml simple) module is a
DOM-style parser built using SSAX, though it returns SXML instead of DOM
objects.

7.20.3.2 Implementation

(sxml ssax) is a package of low-to-high level lexing and parsing
procedures that can be combined to yield a SAX, a DOM, a validating
parser, or a parser intended for a particular document type. The
procedures in the package can be used separately to tokenize or parse
various pieces of XML documents. The package supports XML Namespaces,
internal and external parsed entities, user-controlled handling of
whitespace, and validation. This module therefore is intended to be a
framework, a set of “Lego blocks” you can use to build a parser
following any discipline and performing validation to any degree. As an
example of the parser construction, the source file includes a
semi-validating SXML parser.

SSAX has a “sequential” feel of SAX yet a “functional style” of DOM.
Like a SAX parser, the framework scans the document only once and
permits incremental processing. An application that handles document
elements in order can run as efficiently as possible. Unlike a
SAX parser, the framework does not require an application register
stateful callbacks and surrender control to the parser. Rather, it is
the application that can drive the framework – calling its functions to
get the current lexical or syntax element. These functions do not
maintain or mutate any state save the input port. Therefore, the
framework permits parsing of XML in a pure functional style, with the
input port being a monad (or a linear, read-once parameter).

Besides the port, there is another monad – seed. Most of
the middle- and high-level parsers are single-threaded through the
seed. The functions of this framework do not process or affect
the seed in any way: they simply pass it around as an instance of
an opaque datatype. User functions, on the other hand, can use the seed
to maintain user’s state, to accumulate parsing results, etc. A user
can freely mix their own functions with those of the framework. On the
other hand, the user may wish to instantiate a high-level parser:
SSAX:make-elem-parser or SSAX:make-parser. In the latter
case, the user must provide functions of specific signatures, which are
called at predictable moments during the parsing: to handle character
data, element data, or processing instructions (PI). The functions are
always given the seed, among other parameters, and must return the
new seed.

From a functional point of view, XML parsing is a combined
pre-post-order traversal of a “tree” that is the XML document itself.
This down-and-up traversal tells the user about an element when its
start tag is encountered. The user is notified about the element once
more, after all element’s children have been handled. The process of
XML parsing therefore is a fold over the raw XML document. Unlike a
fold over trees defined in [1], the parser is necessarily
single-threaded – obviously as elements in a text XML document are laid
down sequentially. The parser therefore is a tree fold that has been
transformed to accept an accumulating parameter [1,2].

Formally, the denotational semantics of the parser can be expressed as

The present file also defines a procedure that parses the text of an XML
document or of a separate element into SXML, an S-expression-based model
of an XML Information Set. SXML is also an Abstract Syntax Tree of an
XML document. SXML is similar but not identical to DOM; SXML is
particularly suitable for Scheme-based XML/HTML authoring, SXPath
queries, and tree transformations. See SXML.html for more details.
SXML is a term implementation of evaluation of the XML document [3].
The other implementation is context-passing.